Programmable Shaders December 25, 2006 RenderMan & Its Shading - - PDF document

Programmable Shaders

December 25, 2006

RenderMan & Its Shading Language

Key Idea of a Shading Language

• Image synthesis can be divided into two

basic concerns

– Shape: Geometric Objects, Coordinates, Transformations, Hidden-Surface Methods… – Shading: Light, Surface, Material, Texture, …

• Control shading not only by adjusting

parameters and options, but by telling the shader what you want it to do directly in the form of a procedure

Pixar’s RenderMan

• Separation of Modeling and Rendering

– RenderMan serves as the interface.

• Scene = Shape + Shading
• The power of RenderMan is in the

shading part.

Example #1: Shape Description

#include <ri.h> RtPoint Square[4]={{.5,.5,.5},{.5,-.5,.5}, {-.5,-.5,.5},{-.5,.5,.5}}; Main(){ RiBegin(RI_NULL); RiWorldBegin(); RiSurface(“constant”, RI_NULL); RiPolygon(4, RI_P, (RtPointer)Square, RI_NULL); RiWorldEnd(); RiEnd(); }

Example #2: Shading

• Colorspheres.c in the BMRT/examples

folder.

Building and Running

• Must have the following:

– Header file: ri.h – Link library – A renderer

• The program generates a “RenderMan

Interface” file, but doesn’t render it

– So that you may pick a renderer that matches the graphics power of your machine.

Pixar’s RenderMan

Rman Geom Code cc Rman Program Rman Shader .sl Shader (slc) Byte-code Shader .slc RIB File .rib render Program (rendrib) TIFF image Rman texture Image File txmake

RenderMan Interface Spec

• Where do you find the meaning of the

arguments to those Ri…() functions?

– Check the spec! (available directly from Pixar). – Appendix G contains a quick reference.

http://www.pixar.com/renderman/developers_corner /rispec/rispec_pdf/RISpec3_2.pdf

Shading Language

• Many types of shaders are possible:

– Light source shaders – Surface shaders – Atmosphere shaders – Volume shaders…etc.

• We will discuss only the surface

shaders.

Shader Writing

• Global variables: (from Table 14.2 of

The RenderMan Companion book)

– Camera position, surface color/opacity, surface position/normal, texture coordinates…etc. – Must output: color of light from surface,

• pacity of surface.
• Many built-in operators (Table 14.4) and

functions (Ch.15, Tables 15.1-15.2).

Example: Plastic Surface

surface plastic (float Ka = 1, Kd = 0.5, Ks = 0.5, roughness = 0.1; color specularcolor = 1) { normal Nf = faceforward (normalize(N),I); Ci = Cs * (Ka*ambient() + Kd*diffuse(Nf)) + specularcolor * Ks*specular(Nf,-normalize(I),roughness); Oi = Os; Ci *= Oi; }

RenderMan’s Shader

• Phong shader

surface phong( float Ka = 1, Kd =1, Ks = 0.5; float roughness = 0.1; color specularcolor = 1; ) { normal Nf = faceforward( normalize(N), I ); vector V = -normalize(I); color C = 0; illuminance( P ) { vector R = 2*normalize(N)* (normalize(N) . normalize( L )) - normalize( L ); C += Ka*Cs + Kd*Cs*( normalize(N) . normalize(L) ) + Ks*specularcolor* pow(( R . V ), 10); } Ci = C*Cs; }

RenderMan’s Shader -2

• Attaching to the RIB file

*********** AttributeBegin Translate 0 -1.5 0 Rotate 20 0 0 1 Color [ 0.8 0.0 0.0 ] Surface "phong" "Ka" [.1] "Kd" [.8] "Ks" [1] "roughness" [0.1] "specularcolor" [1 1 1] Basis "bezier" 3 "bezier" 3 PatchMesh "bicubic" 13 "nonperiodic" 10 "nonperiodic" "P" [1.5 0 0 1.5 0.828427 0 0.828427 1.5 0 0 1.5 0 -0.828427 1.5 0 -1.5 0.828427 0 -1.5 0 0 -1.5 -0.828427 0 -0.828427 -1.5 0 0 -1.5 0 0.828427 -1.5 0 1.5 -0.828427 0 1.5 0 0 1.5 0 0.075 1.5 0.828427 0.075 0.828427 1.5 0.075 0 1.5 0.075 - 0.828427 1.5 0.075 -1.5 0.828427 0.075 -1.5 0 0.075

• 1.5 -0.828427 0.075 -0.828427 -1.5 0.075 0 -1.5

0.075 0.828427 -1.5 0.075 1.5 -0.828427 0.075 1.5 0 0.075 2 0 0.3 2 1.10457 0.3 1.10457 2 0.3 0 2 0.3 - 1.10457 **************

Gallery of Shaders

• Procedural textures: e.g., wood
• Bump mapping and displacement

mapping.

• For more, see Ch. 16 of The

RenderMan Companion

• See also the shaders and examples

folders of BMRT.

A Few Stories…

• The shader concept first appears in the

REYES paper.

• The programmable shader first appears in

graphics hardware in UNC’s PixelFlow [Olano97]. Many folks in the PixelFlow team are now at NVIDIA.

• The co-author of PBRT and the creator of

BMRT (Larry Gritz) were cofounders of Exluna

REYES

• From Cook et al. “The Reyes Image

Rendering Architecture” SIGGRAPH 87.

• Subdivide a surface into micropolygons.

BMRT

• A public-domain implementation of

Pixar Photorealistic RenderMan (PRMan).

• Three main components:

– Rendrib: the renderer – Rgl: quick rendering for preview – Slc: shading language compiler

Shading Languages for Graphics Hardware

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Bump Map Example

• Bump mapping simulates detail with a

surface normal that varies across a surface

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RenderMan Example

displacement lumpy ( float Km = 1, frequency = 1, maxoctaves = 6; string shadingspace = "shader"; float truedisp = 1;) { point Pshad = transform (shadingspace, frequency*P); float dPshad = filterwidthp(Pshad); float magnitude = fBm (Pshad, dPshad, maxoctaves, 2, 0.5); N = Displace (normalize(N), shadingspace, Km*magnitude, truedisp); }

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Cg Example

f2fb DiffuseBumpPS(v2f IN, uniform sampler2D DiffuseMap, uniform sampler2D NormalMap, uniform float4 bumpHeight) { f2fb OUT; float4 color = tex2D(DiffuseMap); //fetch base color //fetch bump normal float4 bumpNormal = expand(tex2D(NormalMap)) * bumpHeight; //expand iterated light vector to [-1,1] float4 lightVector = expand(passthrough(IN.LightVector)); //compute final color (diffuse + ambient) float4 bump = uclamp(dot3_rgba(bumpNormal.xyz, lightVector.xyz)); OUT.col = color * bump; return OUT; }

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